The present invention relates to providing a heat resistant cast alloy having high creep-rupture strength and high resistance to cementation, adapted for use at as high a temperature range as around 1,100.degree. C.
As well known, prior-art materials of heat resistant cast alloy include HK40 (0.4% C, 25% Cr and 20% Ni, balance Fe) and HP40 (0.4% C, 25% Cr and 35% Ni, balance Fe), which have been utilized as heat resistant materials of apparatuses in petrochemical industries. Especially, the alloy HK40 prides itself in its years achievements in use as reformer tubes for use under 1,000.degree. C. If it is used as cracking furnace tubes which are heated above 1,000.degree. C., however, it undergoes deterioration in the creep-rupture strength accompanying the coarse-growing of carbides, and with notable sign of cementation. Such tubes required early relacement. These drawbacks found to be further worsened, as the operating temperature approaches around 1,100.degree. C., have left as intractable problems.
At such high temperatures as around 1,000.degree. C., the C deposited on the inside surface of the tube during the cracking of naphtha readily diffuses into the inside surface, causing cementation, and observed in addition thereto are primary carbides spheroidizing and the deposited secondary carbides undergoing early growth, resulting in deterioration in the creep-rupture strength.
On the other hand, the alloy HP40, although stabler than the aforementioned alloy HK40 in the operating temperature range around 1,100.degree. C., in fact, can hardly deal with the deterioration in the resistance to cementation and that in the creep-rupture strength under such circumstances as above described.